This invention relates to the generation of special video effects and particularly to combining selected portions of multiple video images in three dimensions as determined by respective key signals.
Various systems and techniques exist for transforming, combining and/or otherwise manipulating video images, particularly for television. Such systems include three-dimensional image transforming, processors, combiners, and computer graphics image synthesizers that execute hidden surface algorithms.
There are digital optics special effects systems in the television field which modify and manipulate video images at real time video rates. One such system provides transformations of video images, including perspective, at real time video rates. Such system is manufactured by Ampex Corporation, Redwood City, Calif., under the name ADO digital special effects system, and is described in the Ampex Digital Optics Service Manual No. 1809550, issued November, 1983. This transformation system and its manner of operation are also generally described in Bennett et al. U.S. Pat. No. 4,472,732. Effects such as changes in picture size and position are also produced by the system transformations.
A digital combiner for combining images as transformed in this manner is also known, being also manufactured by Ampex Corporation. It is known as an ADO Concentrator combiner, Part No. 1464600, and is described in a service manual catalog No. 1809632-01, issued by Ampex Corporation in April 1984. Such combiner contemplates manipulation of multiple video images utilizing a video effect or technique known as a video channel combine. A "combine" is effected by combining two or more channels of video signals in such a way as to combine scenes or images corresponding to the various signals. For example, a combine may display a first video image from one channel in front of a second video image from a second channel, which in turn is in front of a third video image from a third channel, etc. The combine may appear over a selected background, such as black or gray or a selected color.
Such combiner provides the video images of the various channels in the combine with selected degrees of transparency. This allows images with lower priorities, that is, images which would be hidden behind another image, to show through; i.e., to be partially visible due to the transparency of the image or images in front. In a further effect, the images of any channel also can be dimmed, if desired, to the background. The dimming effect is enhanced by a light source feature which highlights selected images or planes while dimming others. The determination of the transparency and dimness of the channels, like the changes in priority of the channels, can be programmed to occur automatically, and is made on a video field-by-field basis. In addition, soft edge keying with a minimum of aliasing is accommodated, if a soft key is introduced by the signal system. The processes of designating priority, transparency and dimness are readily achieved by the combiner, because the system processes the key signals rather than the video images themselves.
The combiner, when employing at least four channels, can produce two independent combines simultaneously, with each combine being the result of combining two video images from two channels. Any one of the four channels may be selected for combination with any other channel to provide a combine. Each combine process is controlled by one of two different user operated control panels, with each combine utilizing different pairs of video images of the four video channels.
Video channel combines are produced in the combiner by digitally processing binary data samples corresponding to picture elements of an image. The signals in respective channels to be combined are synchronized by appropriate clock and timing signals so that data samples corresponding to the same discrete location on the viewing screen arrive at processing stations in the combiner during the same cycle of processing clock signals. The data samples are combined to produce composite data samples which correspond to a combined or composite video image to be displayed at the same temporal and spatial location on the viewing screen. Data samples are combined in the course of processing by taking a preselected portion of the value of each sample and adding the respective portions.
In the combiner, two or more channels of digital video data samples are supplied directly to a cutter by a signal system. The cutter performs the process of reducing (i.e., "cutting") the magnitude of the binary data samples to some portion of their original value. Each respective channel also supplies associated boundary key data to respective keyer means which, in turn, supply respective processed key signals indicative of the portion that the data samples of each respective channel are to be cut. The processed key signals are coupled to the cutter. The multiple channels of cut video signals are fed from the cutter to an adder. The adder sums the cut video signals to produce one or more composite video signals corresponding to respective composite video images. The composite video signals are fed back to the signal system for conversion to a conventional analog composite video signal. The signal system may comprise a number of the aforesaid ADO systems.
Although such combiner combines signals that have undergone three-dimensional transformation, it does not combine in three dimensions. Rather priority is assigned for an entire video signal, irrespective of the apparent depth perspective.
Three-dimensional effects are known in computer graphics. See, for example, Foley, J. D., and S. Van Dam, "Fundamentals of Interactive Computer Graphics," Addison-Wesley, 1982, and Newman, W. M. and R. F. Sproull, "Principles of Interactive Computer Graphics," Second Edition, McGraw-Hill, 1979. The latter, in Chapter 24, particularly at pages 369 to 370 and 372 to 373, describes a depth-buffer algorithm and scan-line coherence algorithms for hidden-surface elimination. The objects to be displayed are formed of respective polygons. All opaque surfaces are kept track of in three dimensions. The dimensions are in a screen coordinate system. For each polygon within a standard view box, the depth (in screen coordinates, not true depth) corresponding to each pixel on the viewing screen is compared with the depth previously stored for that pixel in a depth buffer, and if it is less than the stored depth, the stored depth is changed to the depth for that polygon and the picture intensity signal for that polygon at that pixel is substituted for the picture intensity signal previously stored. This is repeated for all polygons. This may be done a line at a time.